Info

Source: After McClurkin, 1953.

The least permeable layer may be hardpan or simply a silty layer underlying a loamy surface soil.' For the Atlantic coastal plain from North Carolina to North Florida, SI for longleaf pine

Figure 5.3 Shortleaf pine site index for treeless soils. Where no trees exist for which to measure height and age, curves showing variation in subsoil clay content, steepness of slope, and surface soil depth provide approximations.

depended mostly on the subsoil, with the SI increasing as subsoil texture becomes finer. Here, theoretically, moisture equivalent (a readily duplicated laboratory measure that approximates the percent of the weight of water 24 hours after a soaking rain in a soil sample) rather than soil texture, is the principal criterion involved in establishing SI. The direct relationship of moisture and texture and the convenience of textural classification by simply feeling a pinch of soil with the fingers makes the use of texture a practical tool for the practicing forester. Moisture equivalent plays a role because subsoils furnish water, the amount depending on the proportion of silt and clay in the

Figure 5.4 Plantations of slash pine were established specifically to provide for naval stores operations, beginning in the 1930s. Highest-quality sites, in this case SI 80, were chosen. The 23-year-old stand had been periodically prescribed burned to reduce the fire hazard and to facilitate gum collection from the chipped faces of the trees. (authors' collection)

Figure 5.4 Plantations of slash pine were established specifically to provide for naval stores operations, beginning in the 1930s. Highest-quality sites, in this case SI 80, were chosen. The 23-year-old stand had been periodically prescribed burned to reduce the fire hazard and to facilitate gum collection from the chipped faces of the trees. (authors' collection)

horizon sampled. Thus, a table listing SI for locales with a sand subsoil is 64; for a sandy clay, 72; and for a heavy clay, 77.

For longleaf pine in the southeastern coastal plain, SI improves with increasing latitude and with depth to mottling in the soil. Mottling occurs in poorly drained soil as a result of oxidation, hydration, and reduction of the iron elements that coat the particles of soil. The three chemical reactions take place as oxygen becomes available or diminishes with rising and lowering amounts of water in the soil. Rust red from iron oxidation, yellow from hydration, and bluish-gray from reduction of the ferrous coatings of soil particles mix to give the mottled colors.

For slash pine, SI varies from 75 to 90, with the quality of the site based solely on soil texture, the wide range likely due to soil drainage which, in turn, varies with the proportion of sand, silt, and clay in the soil. SI along creeks and in flatwoods is higher than at more-elevated situations where rainwater drains rapidly.

Figure 5.5 On difficult planting sites like this, erosion control measures, including mulching, might be required. Even unaided, some shortleaf pines and broadleaf cover have become established.

Shortleaf pine SI also relates to soil depth and consistency of subsoil and these, in turn, depend on soil moisture and drainage. A very friable subsoil that begins six inches below the surface of the soil has an SI of 66, a very plastic subsoil at the same depth is charted at 48, while a very friable subsoil 18 inches below the land's surface has an SI of 71.2

Fastest growth for shortleaf pine takes place in the deepest surface soil, the most friable subsoil, and where the subsoil clay content measures less than 20%. Slope also influences growth rate; seedlings grow best on level land. Some species' drought resistance has been attributed to their capacity to absorb water when moisture is limited, to retain moisture in its needles during drought, and to maintain high solute concentrations in the foliage when recovering from drought.3 Although SI theoretically is not related to stocking density, spacing for loblolly pine in plantations showed a gain of 6 to 8% for trees planted 12x15 feet over those at 10-foot spacing.4

Soils formed from the same parent materials, developed under similar conditions, and supporting similar vegetation, have SIs for loblolly pines that depend on soil erodability. SI decreases as much as 10 units with each increasing soil erosion class: class 0, no erosion; class 1, less than 25% of a horizon removed; class 2, 25 to 50% of a horizon removed; class 3, 50 to 75% of a horizon removed; class 4, more than 75% of a horizon removed. Under noneroded virgin soil conditions, SI measures between 90 and 105, and about 50 if most of the surface layer has eroded away.

Multiple Regression

Multiple Regression techniques for relating certain soil properties with the rate of height growth in the Piedmont province found SI for loblolly pine to range from 32 on subsoil described as very plastic and within 2 inches of the surface soil to 91 for very friable subsoil under 18 inches of surface soil. In the Atlantic and Gulf Coastal plains, site potential increases with progressively poorer drainage, irrespective of subsoil texture. However, with drainage constant, SI increases as subsoil becomes finer textured. Depth to the subsoil influences productivity as much as 30 SI units in progressing from 6 to 42 inches.5

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